A leading process for making high-quality glass substrates with pristine surface quality for LCD displays is the fusion down-draw process. The forming device (“forming body” hereafter) in a fusion down-draw process, typically called an isopipe, is illustrated in FIG. 1(PRIOR ART). The forming body 100 illustrated in this figure comprises a trough-shaped part 103 and a wedge-shaped part 107. A glass melt is introduced into the trough-shaped part through an inlet tube 101. A stream of glass melt is allowed to flow over each side of the trough-shaped part, down on both sides of the trough-shaped part and wedge-shaped part, forming a glass ribbon on each side surface of the forming body. The two glass ribbons join at the bottom of the wedge-shaped part 109, typically called root of the forming body, where they fuse together to form a single glass ribbon 111 having two pristine surfaces that have not been exposed to the forming body surface. The glass ribbon 111 is then typically drawn below the root 109 into the desired thickness and allowed to cool to form a rigid glass sheet. As shown, the glass ribbon 111 is drawn in a downward direction 113. The temperature of the glass ribbons from the top of the forming body to immediately below the root are very carefully controlled in order to repeatedly make glass sheet products with desired thickness, thickness homogeneity, and other desired physical properties.
The forming body typically comprises one or more refractory blocks made of materials such as zircon ceramic. The dimensional and geometrical stability and side surface quality of the forming body has significant impact on the quality of the glass sheet product produced.
Thermal gradient in the forming body material can cause stress inside the bulk and/or near the surface of the forming body. Overly high stress can lead to surface cracking, dimensional and geometrical distortion, and even breakdown, of the forming body. Therefore, the thermal gradient the forming body experiences during its life has to be carefully controlled.
Therefore, there exists a need for a method and apparatus that effectively controls the thermal environment of a forming body during a fusion draw process.
The present invention satisfies this and other needs.